Electron transport and ultrafast spectroscopic studies of new methanofullerenes bearing a heteroatom in the exohedral chain

Fullerene derivatives (C60 and C70) have been widely used in excitonic solar cells due to their exceptional electron accepting properties and low reorganization energies. In recent years, a wide variety of fullerene-based n-type materials have been developed, showing tremendous potential in the field of organic photovoltaics. However, only a few studies have been performed on heteroatom bearing methanofullerenes and their opto-electronic properties for use as n-type organic semiconductor materials. In the present study, we report the synthesis of two mono-substituted methanofullerene derivatives, i.e. , C60-Th (Product 1) and C60-TPA (Product 2), with a heteroatom (S and N, respectively) in the exohedral chain attached via an ethylene linker to the cyclopropane ring, and a comprehensive study of their photophysical and electrochemical properties. The methanofullerene derivatives have been synthesized using an amine-assisted cycloaddition (AACA) reaction method. The structures of the synthesized products were established via different spectroscopic techniques. Reasonable quenching efficiencies were observed with respect to the fluorescence emission in mixtures with the donor polymer P3HT using both methanofullerenes. The electron transport properties were evaluated through fabricating electron-only devices, and 10 and 6 times higher mobilities were found compared to PC61BM for Products 1 and 2, respectively. Finally, the charge transfer properties were evaluated in mixtures with P3HT via transient absorption spectroscopy to study the ultrafast charge separation and formation of long-lived charge-separated states. The study suggests that both the products show excellent electron transport properties and the formation of longer-lived charge-separated states in mixtures with the donor polymer for use as n-type materials for organic solar cells. Heteroatom-bearing methanofullerenes with exciting photophysical and electron transport properties are shown to be an addition to the variety of fullerene derivatives for use as n-type materials in organic electronics.